1. Field of the Inventions
The present inventions are related to image forming devices and, more specifically, to image forming devices having pick mechanisms.
2. Description of the Related Art
An image forming device, such as a printer, copier or facsimile machine, typically includes a feed mechanism that supplies substrates onto which images are formed (referred to generically as xe2x80x9csheetsxe2x80x9d) to the image forming region of the image forming device. Many image forming devices include a tray that stores a stack of sheets. A pick mechanism may be used to pick the top sheet off of the stack and advance the sheet to the feed mechanism.
One type of conventional pick mechanism includes a rotating pick roller that is brought in to and out of engagement with the top of the stack at the appropriate time through use of a pick arm. One end of the pick arm is mounted on a drive shaft that is supported by the chassis of the image forming device. The pick roller is supported on the other end of the pick arm. The pick arm moves between an idle position, where the pick roller is disengaged from stack, and a pick position, where the pick roller engages the top sheet in the stack. The drive shaft also supports a drive shaft gear that drives a series of idler gears on the pick arm which, in turn, drive the pick roller. The pick roller frictionally engages the top sheet and urges the sheet against a stop that is positioned at the front corners or center of the tray. This causes the top sheet to buckle and separate from the remainder of the stack prior to being urged forward to the feed mechanism. One example of such a pick mechanism is illustrated in U.S. Pat. No. 5,527,026.
During the pick process, the magnitude of the normal force exerted onto the pick roller must be maintained within a predefined range so that the pick roller will properly engage the top sheet in the stack. If the magnitude of the normal force is too low, the pick roller will not be able to frictionally engage the top sheet. If the magnitude of the normal force is too great, multiple sheets may be fed and back tension will be created, which can lead to banding and other printer problems. The normal force exerted onto the pick roller in a conventional pick mechanism is equal to the weight of the pick arm plus the force resulting from the torque applied to the pick arm by the drive shaft through the drive shaft gear prior to movement of the top sheet, less any force associate with the counter balance spring in those instances where such a spring is used to bias the pick arm to the idle position.
The inventors herein have determined that one disadvantage associated with this type of conventional pick mechanism is that it can only be effectively used with smaller capacity trays, i.e. about a 100 sheet stack of 20 lb. paper or a stack of other media that is about 10 mm (0.4 inch) in height. In a larger capacity tray, i.e. about a 250 sheet stack of 20 lb. paper or a stack of other media that is about 25 mm (1.0 inch) in height, the normal force on the pick roller is insufficient to properly pick the top sheet from the stack when the tray is full. The reason is as follows.
Regardless of tray capacity, it is preferred that the pick arm move within a range of motion that is typically between about 1 degree (tray full) to about 21 degrees (tray essentially empty) measured from the top of the stack. A pick arm used in conjunction with a larger capacity tray must be longer than that used in conjunction with a smaller capacity tray in order to maintain the 21 degree angle when the larger capacity tray is essentially empty. Increasing the length of the pick arm, however, decreases the magnitude of the normal force imparted on the pick roller by the drive shaft. As a result, the total normal force imparted on the pick roller when the tray is full will be below the level sufficient to properly pick the top sheet from the stack.
One proposed solution to the problems associated with the use of a conventional pick mechanism in combination with a larger capacity tray is to simply increase the weight of the pick arm, thereby increasing the magnitude of the normal force on the pick roller when the tray is full. The present inventors have determined that this proposed solution is unsatisfactory because the additional weight increases the normal force on the pick roller when the tray is close to empty to a level that results in multiple sheet feeds and excessive back tension. Another proposed solution is to increase the size of the drive shaft gear, thereby increasing the torque applied to the pick arm prior to the buckling of the top sheet. The present inventors have determined that this proposed solution undesirably increases the overall size of the pick mechanism.
Accordingly, one object of the present inventions is to provide apparatus that avoids, for practical purposes, the aforementioned problems in the art. In particular, one object of the present inventions is to provide a pick mechanism that may be used in combination a larger capacity tray.
In order to accomplish some of these and other objectives, a pick mechanism in accordance with one embodiment of a present invention includes a pick roller, a pick arm supporting the pick roller and an actuation system adapted to move the pick arm from an idle position to a pick position and impart additional normal force to the pick roller. The magnitude of the normal force, which is greatest when the tray is full, decreases as the size of the stack in the tray decreases. The present pick mechanism therefore allows the pick roller to both frictionally engage the top sheet in a full relatively large tray and function properly when the tray is close to empty. In a preferred embodiment, the additional force will only be applied during the pick process. As such, deformation (or xe2x80x9ccreepxe2x80x9d) of the pick arm and other parts will be minimized.
The above described and many other features and attendant advantages of the present inventions will become apparent as the inventions become better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings.